1. Field of the Invention
This invention relates to a radiation image read-out method in which a stimulable phosphor sheet carrying a radiation image stored thereon is exposed to a stimulating ray which causes it to emit light in the pattern of the stored image, and the emitted light is photoelectrically read out by a photodetector, and apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to such radiation as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to a stimulating ray such as a visible ray, light is emitted from the phosphor in the pattern of the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phoshor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and read-out system. Specifically, the stimulable phosphor formed on a sheet is first exposed to a radiation transmitting through an object to have a radiation image stored therein, and is then scanned with a stimulating ray such as laser beam which causes it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce an visible image on a recording medium such as photographic light-sensitive material or on a display such as cathode ray tube (CRT).
This radiation image system using the stimulable phosphor sheet is advantageous over the conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing and diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desirable density regardless of the amount of exposure of the phosphor to the radiation by reading out the emitted light with an appropriate read-out gain and converting it to an electric signal to reproduce a visible image on a recording medium or a display. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing and diagnostic purposes. This is very advantageous in practical use.
As mentioned above, in the radiation image system using a stimulable phosphor, deviation of the level of the radiation energy stored in the stimulable phosphor from a desired level can easily be compensated by setting the read-out gain to an appropriate value when photoelectrically reading out the light emitted from the stimulable phosphor upon stimulation thereof. Therefore, the quality of the reproduced radiation image is not adversely affected by a change in radiation dose due to fluctuating tube voltage or MAS value of the radiation source, a variation in the sensitivity of the stimulable phosphor or the photodetector, a change in radiation dose according to the condition of the object, or a change in the radiation transmittance according to the object etc. Further, it is possible to obtain a desirable radiation image even when the radiation dose to the object is reduced. Further, it is possible to obtain a radiation image having high image quality of high contrast, high sharpness and low noise etc. by once converting the light emitted from the stimulable phosphor into an electric signal, and processing the electric signal as desired. Particularly, when the radiation image is used for medical diagnosis, it is possible to obtain a radiation image processed in accordance with the portion of a human body such as the heart, the chest etc. and improve the diagnostic efficiency and accuracy.
However, in order to eliminate various influences based on the fluctuation of radiographic conditions and/or obtain a radiation image having a high image quality or a high diagnostic efficiency and accuracy, it is necessary to investigate the image input condition of the radiation image stored on the stimulable phosphor sheet or the image input pattern, which is determined by the radiographic method such as portion image (e.g. chest and abdomen), plain image or contrasted image radiographing, before reproducing the radiation image to a visible image, and appropriately adjust the read-out gain or appropriately process the electric signal based on the investigated image input condition or the image input pattern. The image input condition and the image input pattern will hereinafter be simply referred to as the image input information when they should be expressed generically. It is also necessary to determine the scale factor to optimize the resolution according to the contrast of the image input pattern.
The investigation of the image input information may be conducted prior to the visible image reproduction by use of the method disclosed in U.S. Pat. No. 4,284,889, which is based on the observation that the amount of light instantaneously emitted from the stimulable phosphor sheet upon exposure thereof to a radiation is proportional to the amount of the radiation energy stored in the stimulable phosphor. In this method, image input information is investigated by detecting the instantaneously emitted light, and an appropriate signal processing is conducted based on the image input information in order to obtain a visible radiation image having a high diagnostic efficiency and accuracy. With this method, since it is possible to appropriately adjust the read-out gain, select an appropriate scale factor, or conduct an appropriate signal processing, a radiation image suitable for viewing and diagnostic purposes can be obtained regardless of fluctuation of the radiographic conditions. However, since the radiation image input section and the radiation image read-out section are usually substantially spaced apart from each other, a signal transfer system must be formed therebetween, necessitating a complicated apparatus and a high cost.
Further, U.S. Pat. No. 4,276,473 discloses a method of estimating the image input condition or image input pattern of a radiation image stored in the stimulable phosphor by positioning a non-stimulable phosphor in the vicinity of the stimulable phosphor sheet, and detecting the light emitted from the non-stimulable phosphor upon exposure thereof to a radiation by use of a photodetector. However, this method also has the same drawback as that of the method disclosed in aforesaid U.S. Pat. No. 4,284,889. Further, since the stimulable phosphor for recording the radiation image in not used to detect the image input information, it is impossible to correctly investigate the image input information.